The Hebrew for that part of the verse is:
כִּי מַה-טּוּבוֹ, וּמַה-יָּפְיוֹ
The words "tov" (good/goodness) and "yafi" (beauty) have the suffix "וֹ", which is third-person singular masculine. This suggests that the goodness and beauty being talked about belong to God (but see below for another idea), and not to some unnamed "they".
A more literal translation of this phrase than either of the ones quoted would be:
For what is his goodness, what is his beauty.
When ESV says "how great" it is filling in; the author is not asking a true question (what?) but is writing poetically, like when somebody says (while looking straight at something) "what is this I see before my eyes?".
I note that Mechon Mamre, using JPS 1917 as input, translates this phrase thus:
For how great is their goodness, and how great is their beauty!
I do not know why they, like NLT, use the plural, but I have a speculation: given the context of the previous verse, perhaps instead of talking about God the psalmist is talking about the nation of Israel. While v. 16 starts out in the plural ("their God"), it proceeds to the singular כְּצֹאן עַמּוֹ (like a flock of his people -- singular "people"). Since the goodness and beauty of whomever is followed by a description of the bounty to fall to members of the nation, it's possible that the "he" is the nation of Israel and not God.
Here is the Mechon Mamre translation of these two verses, with number indicated for key words. The text in brackets is not in the Hebrew directly but is implied, so interpretation of number is open.
And the LORD their (pl) God shall save them (pl) in that day as the flock (s) of His people (s); for [they shall be] as the stones (pl) of a crown (s), glittering over His land (s).
For how great is their (s) goodness, and how great is their (s) beauty! Corn shall make the young men flourish, and new wine the maids.
Conclusion: the referent is definitely singular masculine. Usually the singular masculine being praised in psalms is God, but there is room to interpret here as the nation. The Hebrew text alone is not unambiguous. A study of biblical poetic forms might shine more light on the question (but is beyond my current knowledge).
Please note: This answer was written for a neutral, academic audience
and is not intended to be interpreted in the context of a religious
belief or doctrine.